Rename users table to tracked_profiles ahead of adding accounts table
Renames the SQLite `users` table to `tracked_profiles` and the `User`
model to `TrackedProfile` to avoid confusion when we add a separate
`accounts` table for authenticated Bluesky users. Includes a migration
to rename the table and its index, and updates all imports, raw queries,
and comments across the codebase.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>